Field of the Invention
This invention relates to a sheet post-processing device and an image forming apparatus. More specifically, this invention relates to a sheet post-processing device and an image forming apparatus which folds a conveyed sheet at two or more positions.
Description of the Related Art
Conventionally, sheet post-processing devices which convey sheets and perform post-processing on the sheets being conveyed are used. Such a sheet post-processing device is used for performing post-processing on sheets on which images were formed, in an image forming apparatus, for example.
Some kind of sheet post-processing device can fold a sheet at two or more positions by using a pair of folding rollers or the like, as post-processing. For example, to perform Z-folding, outer threefold, inner threefold, or the like, the first folding is performed at the first folding position, and the second folding is performed at the second folding position.
The Document 1 below discloses a structure of a post-processing device which performs printing or stapling at the folding position. The device detects the printing or stapling to determine the folding position.
The Document 2 below discloses a sheet folding device which determines the first folding position, based on conveying rollers at the upstream side and at the downstream side of the folding location and a sensor installed at downstream of the downstream conveying roller, to improve the accuracy of the first folding position. The second folding position is decided by detecting the leading end of the first folded portion, by using a sensor installed at a branched path.
The Document 3 below discloses a structure of a sheet process device which performs the second folding by feeding the upstream sheet, pinching the sheet after the first folding by a pair of the second rollers which is installed interposing the conveying path. The sheet process device detects an edge face of the sheet by a sensor installed at upstream of the pair of the second rollers. In response to the result, the first folding position and the second folding position are decided.                [Document 1] Japan Patent Publication No. 2004-238201        [Document 2] Japan Patent Publication No. 2014-118241        [Document 3] Japan Patent Publication No. 2007-22693        
When a sheet post-processing device performs folding, an error of the folding position may generally occur. Such an error is caused by variation in the feeding amount of rollers which move a sheet to the folding position, a deflection of a sheet which occurs in a conveying path, variation in the friction coefficient of a folding roller, variation in stiffness which occurs by the difference in quality of sheet material and the difference in basis weight, variation in the friction coefficient of a sheet which occurs by quality of sheet material, or the like.
Hence, when post-processing is performed by a mode of Z-folding or the like, in which a sheet is folded at two positions in series, by using such as a method of the sheet folding device disclosed by the Document 2, problems may be developed.
FIG. 15 is for explanation pertaining to an error of the folding position, when Z-folding is performed conventionally.
Referring to FIG. 15, the occurrence of an error of the folding position when Z-folding is performed will be explained, for example. More specifically, in this instance, the first folding is performed after detecting the location of a sheet, on the basis of leading end E1 of the sheet. The second folding is performed after detecting the location of the sheet, on the basis of the leading end of the first folding position F1 which was made by the first folding. Then, the error (variation) D2 of the second folding position F2 at which the second folding is performed includes the error D1 of the first folding position F1 occurred when the first folding was performed. Since the errors are accumulated for the second folding position F2, the error D2 of the second folding position F2 is large, when compared to the error D1 of the first folding position.
An effective solution to the problem is not disclosed in the above mentioned Documents 1 and 3.
According to the method disclosed in the Document 3, the location of the end of the sheet is detected, and the first folding position is decided. After that, the second folding position is decided, based on an amount of drawing by the pair of rollers when the first folding and an amount of pushing into the pair of rollers which performs the second folding. Therefore, the error occurred at the first folding position is accumulated in the error of the second folding position.
According to the method disclosed in the Document 1, a mark should be printed on the sheet. Therefore, another problem of degradation in appearances of the sheets is developed. Further, the method wherein the folding position is decided by using a staple as a sign can not be adopted when one sheet is folded.
This invention is to solve the above problems. The object is to provide a sheet post-processing device and an image forming apparatus which can fold a sheet with precision.